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 A065930 (x,y) = (a(n),a(n+1)) are the solutions of (t(x)+t(y))/(1+xy)) = t(4) = 10, where t(n) denotes the n-th triangular number t(n) = n(n+1)/2. 0
 4, 79, 1575, 31420, 626824, 12505059, 249474355, 4976982040, 99290166444, 1980826346839, 39517236770335, 788363909059860, 15727760944426864, 313766854979477419, 6259609338645121515, 124878419917922952880 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS J.-P. Ehrmann et al., Problem POLYA002, Integer pairs (x,y) for which (x^2+y^2)/(1+pxy) is an integer. FORMULA a(n) = 2t(m)a(n-1)-a(n-2)-1, a(0) = m, a(1) = m^3+m^2-1 with m = 4. G.f.: (5x-4)/((1-20x+x^2)(x-1)). MAPLE g := (5*x-4)/(1-20*x+x^2)/(x-1): s := series(g, x, 40): for i from 0 to 30 do printf(`%d, `, coeff(s, x, i)) od: # James A. Sellers, Feb 11 2002 CROSSREFS Cf. A000217 (triangular numbers). Sequence in context: A215843 A048957 A006425 * A018807 A216410 A125710 Adjacent sequences:  A065927 A065928 A065929 * A065931 A065932 A065933 KEYWORD easy,nonn AUTHOR Floor van Lamoen, Nov 29 2001 EXTENSIONS More terms from James A. Sellers, Feb 11 2002 STATUS approved

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Last modified June 5 03:12 EDT 2020. Contains 334828 sequences. (Running on oeis4.)